altera_avalon_timer: disable high-resolution timestamps

On Nios II the same timer peripheral IP block can't function
as a periodic system timer and a high-resolution timestamp source.
A second timer instance with different configuration is required.
Until that is implemented, just return the accumulated cycle count.

Issue: ZEP-1546
Change-Id: If3dcebdc60334bf3aa0ab45ccd82f1b2531b6bc1
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>